It may boil down to this: do we ever really know the people we hold close? And if not, what will the consequences be?

Lori Reuter: It felt awful just not having her there.

At the center of this story...an attractive couple...Shaun Gayle, a famous ex-football player...Chicago Bears...smart and very charming. And Rhoni Reuter, a fitness fanatic...stunning, stylish and...very sweet. Her brothers, Wayde and Thad, adored her as did their wives, Lori and Anna.

Anna: From the minute you saw her, you felt happy just–just being with her.

Lori: Rhoni is the person that you see that you would want to be. She was everything. She would do anything for anybody.

Wayde Reuter: She was very caring, always thought of other people. She thought of her nieces or nephews, everybody, us, called us every night. That's just the way she was.

Even after Rhoni left their hometown of Potosi, Wisconsin, and moved 200 miles to Chicago, she stayed very close to her family, including her nieces and nephews.

Thad Reuter: Christmastime she always wanted to be there for the kids. She wanted to be Santa Claus, handing the presents out to the kids. Watch them open their presents. She just loved Christmas. She loved being with family. She loved going to the kids' events.

Anna: There was a warm tenderness that Rhoni brought everywhere she went.

She had a big heart, with plenty of room for many devoted friends, but perhaps at its center...Shaun Gayle.

The two had met in the late 80's in Platteville, Wisc., where Rhoni went to college and Shaun practiced football at a Chicago Bears training camp. Back then he was a member of the iconic '85 Bears team that won the super bowl. "Da Bears," as they became known, were lionized across the country...and Shaun was a part of all that.

Anna: For the most part, they were just together all the time. They talked every day on the phone. They took trips together.

They made it work...the adored pro athlete and the girl from a small town living in the big city... eventually juggling two jobs–one in the food industry and the other at Macy's...and then, in March 2007, after many years together, Rhoni became pregnant at age 41. She'd finally become a mother...truly a dream for her. She was elated...which was obvious when Wayde and Lori visited Rhoni in Chicago that September.

Wayde Reuter: That was the happiest I've ever seen my sister in my life. And she was just glowing.

Her family thought she might never be able to have children, so...the baby...a girl...was an unexpected gift...due that December. She'd already been named Skylar.

Lori Reuter: So to us, when she got pregnant, that was a miracle. Skylar was gonna be our little miracle baby.

And as far as they knew, Shaun was supportive of Rhoni during the pregnancy.

Thad Reuter: I did ask her once. I said, "How–how's Shaun taking it all? Are you guys going to the doctor together and stuff?" And she said, "Yeah, he comes to the doctor." He went to her ultrasounds and everything.

Rob Stafford, Dateline Correspondent: Engaged in the whole pregnancy.

Thad Reuter: Yeah.

Rhoni was a woman in love...but this was no conventional relationship. Though she'd been with Shaun for close to two decades...they never lived together...and it was unclear whether that would change...even after the baby arrived.

Rhoni’s Sister-in-law: She talked about moving in with him to make it easier for the baby as far as handling the day care. But I don't know where they were at with that. That was the last I had heard.

Rob Stafford: Did she wanna get married?

Anna: Yes, she did.

Rob Stafford: Did he?

Anna: I don't know.

Anna would ask Rhoni how she could stay with a man so long and not live together or be married when she wanted that so much.

Anna: And we always came back to the same thing. She'd be like, "I just love him." I'd say, "What is it about him?" and she said, "I don't know what to tell you, Anna, I just love him."

How did the family feel about their relationship?

Wayde Reuter: I didn't know really that much about Shaun to tell you the truth. All's I know is she loved him to death. That was her soul mate. Her one person she wanted to be with the rest of her life.

In fact, Shaun rarely saw Rhoni's family over all those years.

Thad Reuter: All Rhoni ever really wanted was for Shaun to be a part of our family and to get to know us. She would have loved it if he would have embraced our family and just took the time to get to know us.

Wayde Reuter: It wasn't like she wanted him to come out and visit us every weekend or anything. Just the holidays. And he was invited to every holiday at our place.

Thad Reuter: We just knew that Rhoni said Shaun was a private person and didn't want people bothering him.

Wayde Reuter: She'd say, "He's gone on Christmas.” He was gone. “He's over in Europe,” or whatever doing NFL Europe. She never really talked a lot about him.

Rather strange...for a woman who came from a home where family was so important. Just what was going on? So much more, really, than anyone knew,

Shaun Gayle and Rhoni Reuter...they'd been together for nearly two decades, never lived together or married... and now Rhoni was pregnant with their child. A new chapter for any couple, but for this one, things were far more complicated than they appeared. For starters, there were other women in Shaun's life.

Surprising? That women would find him attractive? Of course not. Who wouldn't like him?

Sports Reporter, Bruce Wolf: Shaun Gayle has always come across as an intelligent, forthright, charming man, and, you know, a guy that you'd like to know.

What's more, says Bruce Wolf, a sports reporter in Chicago for 30 years, Shaun is still a part of football royalty, being a player for the '85 Bears...hometown heroes who still rule...even years later...in a city with many professional sports teams. The '85 Bears' easy win at the super bowl along with their cockiness made them all immortals, Wolf says, including Shaun Gayle...who made a key fumbled punt recovery in the playoffs.

Bruce Wolf: It's the shortest punt return for a touchdown in NFL history

As Shaun's football career was ending, he made a move that polished his image even more. In the mid-90's, Shaun combined his notoriety and college degree in education with his love of sports and Christianity...writing children's books called “Shaun Gayle's Sports Tales,” stories with moral messages based on Christian values. He talked with Bryant Gumbel on the Today Show about why he wrote the books.

Shaun Gayle: So I wanted to get the message out, trying to get across to them.

Shaun Gayle: That perseverance, applying yourself, listening to older people instruct you on how to live your life, understanding what's good and bad, right and wrong.

While Shaun was promoting his books about good and bad, right and wrong, he was also quite the ladies' man.

Bruce Wolf: Professional athletes have fame, power, money. They're good-looking. They're in great shape. They're going to get women.

Shaun was no different. Even though Rhoni was his long time girlfriend, she wasn't the only woman he was seeing over the years. One was Monika Kurowska, a fitness model he met at a Chicago Bears' event. Another was Marni Yang, a divorced mother of three, who was brokering real estate deals for him... and...was an aspiring fitness model as well.

Photographer Jason Stoller: Someone that's promoting sales of fitness equipment, like we see on TV all the time.

Jason Stoller first met Marni during a quick shoot in her garage. Marni's children were there...twins, a girl and boy, 15 at the time...and a younger son, age ten.

Rob Stafford: How was she with her kids?

Jason Stoller: I saw a very in-control mom–protective of her children–that communicated very well and was even really, really liked by her children's friends.

He took more than a thousand photos of Marni during four shoots in 2006 and 2007. Although then nearly 40...Marni was in great shape.

Jason Stoller: Good muscular development that was still feminine without being overly developed.

Jason said Marni was incredibly driven.

Jason Stoller: She was looking for success in real estate. She was looking for success as a fitness model.

Rob Stafford: Was the modeling about ego? Or was it about trying to make money?

Jason Stoller: I think both. I think this woman was trying to make all the money she could for her and her family.

And Jason says Marni really loved the camera.

Jason Stoller: She has fun. She enjoys herself. She pushes herself. She wants to shoot and shoot and shoot and shoot.

Jason's girlfriend, Miranda Bauman, did Marni's make-up.

Miranda Bauman: Well, I was stunned by how tiny she was. So tiny. She couldn't have been more than a size two. She was incredibly energetic and driven, motivated. Had a lot of interests: athletic, sky diving, scuba diving, these kinds of things.

She was also fascinated by police work...even applied twice to be an officer...but was turned down. Still, she was very active in the Chicago police department's neighborhood watch program.

Miranda Bauman: She talked about wanting to make sure her neighborhood was safe and being concerned about her children's safety.

One day, they say, Marni brought up a subject that caught Jason's attention: the Chicago Bears.

Jason Stoller: She alluded that she was involved with–from what she told me, with one of the Bears’ Super Bowl football players.

Jason says the man's name was one he knew well.

Jason Stoller: She mentioned Shaun Gayle, that she was involved in real estate transactions with him and this is who she was seeing.

Jason Stoller: Seeing. From what Marni said, it led me to believe they were intimate. She did indicate to me she knew Shaun was involved with other people that, you know, so obviously, it was clear it wasn't something exclusive.

Marni had shown up at some of the shoots with a man she described as a family friend...and she was, in fact, seeing a few men, but it appeared it was Shaun she was really interested in and wanted a romantic relationship with. In fact, on her last shoot in the winter of 2007, Jason says Marni arrived with an outfit and a request.

Jason Stoller: She said, "I wore this specifically and I would like to get some pictures specifically for Shaun. And it said Chicago Bears on the front. And it said Chicago Bears on her butt. And she wanted to take some pictures in this outfit for Shaun.

It's not clear what...or how much...Rhoni Reuter knew about Marni Yang or the other women in Shaun's life over the years...but she was still his girlfriend...still the woman he took to Bears events. And her family says, he was still the love of her life. But she might have been suspicious because at one point she started receiving anonymous letters...with a warning about Shaun.

Deerfield Police Chief John Sliozis: Describing the fact that he was not only seeing this one particular woman but seeing many women at the same time.

Shaun said the letters were full of lies...the work of someone out to make him look bad...harass him. Who had sent them? And would they be a sign of something far more sinister to come?

Former Chicago Bear Shaun Gayle had been involved with several women at once. His longtime girlfriend, Rhoni Rueter, had been getting letters accusing him of being a womanizer. And now she was pregnant.

Lori Reuter: The last picture I have of her, the baby was moving and she said, "Mom, come and feel my tummy. The baby's moving." I will never forget that.

The last picture of Rhoni? Sadly, yes.

Photos taken when her family visited Rhoni in September, 2007 are precious treasures now. Less than three weeks after this image was taken...Rhoni was dead.

And worse, it was murder. Rhoni had been shot seven times...as she opened the front door of her condo early one October morning.

Thad Reuter: I just remember Dad calling, and he just said, "Your sister's–it was a homicide. Your sister's gone."

Wayde Reuter: That was probably the most angry I've ever been in my life. I was in an uproar, I mean, I just wanted to go down there and get the person. I wanted to grab them myself and do what they did to my sister.

The news was devastating. Rhoni Reuter and her unborn baby were gone. Her whole family was distraught, but no one more than Rhoni's parents...their grief was hard for her brothers to bear.

Wayde Reuter: Nobody should ever have to go through this. Nobody should ever have to see their mom–a mom that's–that was like a mountain for us to turn into a molehill.

Thad Reuter: It was, like, she didn't wanna live either. Dad actually said it, too. "Why couldn't it have been me? I've lived 65 years, you know, why does it have to be our daughter? You should never outlive your children."

The family brought Rhoni back to Potosi to be buried. It seemed everyone came to say goodbye.

Wayde Reuter: And then at the wake, it was just overwhelming–the people. People came in, came in. They just never stopped. It was non–stop for I don't even know how many hours. And it was–it's just hard to see her laying up there on a slab like that, you know? And that's your sister.

Soon after the funeral, Thad, Wayde and their wives drove to Rhoni's condominium outside Chicago to pack her things.

Anna: We walked in and it was stark quiet and cold and empty. And it just–it felt awful. All of her stuff but not her. It was awful.

Lori Reuter: She had all–the baby clothes that people had gotten her were on a great big table next to the kitchen.

Thad Reuter: She's never created any enemies that we know of. And why would someone wanna do this to her? What–what's the reason? And I couldn't come up with any other than she's pregnant and possibly someone wanted, you know, her out of the way. And that was the only conclusion I came to.

It certainly looked that way to police in Deerfield, Illinois...the upscale Chicago suburb where Rhoni lived...known for high test scores and low crime rates. Deerfield hadn't seen a murder in more than 30 years. And now a woman was shot in her own home. Police noticed there was no forced entry.

Rob Stafford: Any sign of robbery?

Police Chief Sliozis: None.

Police didn't know it yet, but something was taken from the scene...something very personal...which would ultimately provide a clue about the motive. Deerfield Police Chief John Sliozis says the fact seven shots were fired said a lot about the crime...and the killer.

Police Chief Sliozis: The sheer number of shots would indicate that this–there was some emotion. There was some–something driving the shooter. It appeared to be personal. In all likelihood the victim could have known the offender.

Lab tests showed all seven bullets came from one 9mm handgun, a weapon that makes a lot of noise. But the neighbor who called 911 said the shots she heard were muffled...perhaps a silencer was used.

Police Chief Sliozis: That it was planned. It was calculated. Took steps to be discreet.

But apparently not discreet enough. Right after the killing, a witness saw someone leave the front entryway...quickly walking to a black car.

Rob Stafford: Description of the suspect?

Police Chief Sliozis: Young man small in stature, dark complexion and with dark hair and seen running or walking at a quick pace across the parking lot.

The suspect was dressed in baggy clothes and looked like a teenager...a black male teenager...but as investigators looked for someone fitting that description, they looked elsewhere, too.

Police Chief Sliozis: You go into it with an open mind. It's that old police adage, "Everybody's a suspect."

...including Rhoni's long time boyfriend, former Chicago Bear, Shaun Gayle. As soon as Shaun, who says he was the father of Rhoni's unborn baby, heard about the murder that day, he came right in to talk with police.

Rob Stafford: What did he say?

Police Chief Sliozis: Obviously he was in–in great distress, emotional, crying, you know, not–not in a physical state of shock, but just really in a kind of state of awe–that it could have happened. Why did it happen? Who could have done this?

At the time of the murder, Shaun told police, he'd been driving about 40 miles north of Chicago to get his hair cut...at a barbershop favored by Chicago Bears players. His alibi checked out...but as police quickly came to believe Shaun was not a suspect in Rhoni's murder...it became clear he would remain central to the case. One woman in his life was dead, and now police were going to take a hard look at the others. A web of tangled relationships was about to unravel...and twisted emotions would be revealed.

The murder of Rhoni Reuter in her own home stunned Deerfield, Illinois...a quiet upscale Chicago suburb. Police ruled out Rhoni's long time boyfriend, Shaun Gayle, as a suspect, but he remained central to the case, particularly because a series of anonymous angry letters about Shaun and other women had arrived at Rhoni's door the day before she was killed. And police learned that similar letters had been sent to other people in Shaun's life as well.

Police Chief Sliozis: They were being sent to other female acquaintances of Shaun Gayle describing the fact that he was not only seeing this one particular woman but seeing many women at the same time, accusing him of using women just for his personal gain and satisfaction and just basically trying to dissuade these other ladies from seeing him.

Dateline has obtained some of the letters, which not only contain angry allegations of his cheating and lying, but also include copies of what are purported to be very personal emails between Shaun and the women...as well as supposed travel itineraries indicating Shaun had taken foreign trips with two different women less than two months apart.

Shaun has said the letters were filled with lies. But did they contain clues to the killer's identity?

One was written in poor English as if by an immigrant. Shaun had immediately thought of Monika Kurowska...who had moved to the states from Poland. She was the fitness model he'd met at a Chicago Bears event and dated for several months.

And that relationship had ended badly in May, 2006. In papers filed in court, Shaun said Monika had repeatedly banged on his window, breaking it. Soon after that, the letters started to arrive...assuming it was Monika sending them, Shaun took out an order of protection against her. But the letters continued. It seemed to Shaun that Monika was violating that order. As a result, she was threatened with deportation.

The harassment went on for more than a year...including emails...alleging his womanizing...sent three months before the murder...to participants in a charity event benefiting abused women. Shaun had been designated the event's celebrity ambassador. At the time he wanted Monika jailed or fined for apparently violating the order of protection. The request...coincidentally...was filed in court the same day Rhoni Reuter died.

But Monika had denied any involvement with the letters and emails...and she was denying any involvement in Rhoni's murder now. Police checked her alibi and confirmed she was training a fitness client at the time of the shooting. In fact, their investigation found she didn't write the letters and had been wrongfully accused by Shaun. Monika was never deported, fined or jailed...and as she was ruled out...police began to investigate someone else...another woman listed in the letters...the real estate agent who'd posed in that little Chicago bear's outfit...a few callers had mentioned her name.

Police Chief Sliozis: Three phone calls from people who gave the name of Marni Yang.

Rob Stafford: Marni Yang?

Police Chief Sliozis: Yes. The information they were leaving was that she would be an individual capable of doing something like this because of the relationship she had with Shaun Gayle.

Rob Stafford: Capable of this kind of murder?

Police Chief Sliozis: Yes.

One caller described Marni as a person with a grudge...and a gun.

Police Chief Sliozis: She knew she had carried guns in the past, had a temperament of revenge or certainly knee-jerk anger responses.

Police Chief Sliozis says a check of court records uncovered more orders of protection, but these involved Marni Yang... including one against her by a married Chicago policeman she'd apparently had an affair with.

Police Chief Sliozis: He had some real concerns about his own safety and wanted to distance himself from this woman.

The officer said Marni had sent a "harassing" letter to his wife threatening his "life, career and marriage" and he feared for "his safety and the safety of his family."

Rob Stafford: Did you ask Shaun Gayle about Marni Yang?

Police Chief Sliozis: Yes. I think he described initially a working relationship with her. That he knew her through some real estate deals that she had brokered for him. As the investigation continued, I think the investigators found out that it was certainly a business relationship, but there was also a dating relationship.

Rob Stafford: At the time of the murder, had there been any confrontation between Marni Yang and Shaun Gayle?

Police Chief Sliozis: No, they were still talking and they were still seeing each other.

In fact, the investigation found the two spent time together at his home the evening before Rhoni was killed...Marni had brought him dinner and stayed for awhile.

Did Shaun Gayle know anything about the history of this woman who was in his life? Investigators were getting an earful.

In the weeks after the murder, detectives were learning from people who knew Marni... about what they described as her obsession with men over the years...and how she acted when she didn't get her way. Her relationship with the married police officer was but one example. Turns out...that behavior went all the way back to her teen years.

Melissa Dessent: If she felt that she wasn't getting the attention that she was looking for, she would get very angry about it.

Melissa Dessent, a close high school friend back in the 80's, says Marni had quite a temper, especially when it came to boys.

Melissa Dessent: She was a very intense person. When she was interested in something, she became somewhat fixated on it. She was very into boys–very into herself, you know, "Everything is about me" type of attitude.

Melissa says on Sunday evenings, Marni and her friends would hang out at a local club...then go out for fast food. But for Marni...the night was just beginning.

Melissa Dessent: She would drive everybody home and this one guy who lived by my house, about a block away, she would drop him off last. And they would have sex in her car and then she would show up at my house at about 1, 2 o'clock in the morning, knock on my window and stand and talk to me through the window for an hour or two at a time.

Boys, sex and obsessive behavior sometimes morphed into stalking, Melissa says. Later when Marni went away to one college...her summer boyfriend went to another...and soon found a new girlfriend. Melissa says Marni didn't like that...not one bit.

Melissa Dessent: She was stalking him long distance. She would drive over on the weekends where he was away at school and just randomly show up. I believe she was also calling him repeatedly and sending him letters as well.

Eventually Marni met her future husband. A young man named Yen Yang. He was a Cambodian refugee who was taken in and cared for by an American woman when he came to the states as a child.

Yen stayed with Marni's family during school breaks...but after he and Marni got engaged...Melissa says he found Marni's diary and what he read inside almost ended their relationship.

Melissa Dessent: Marni had a history of writing down her conquests with boys, and he got quite irate over what he found because when she met him, she had told him that she'd only been with one other guy before him. And it happened to be the guy that she was stalking at the other school.

She says Marni told him she'd made everything up...it was all a fantasy. But Yen was so upset, he called off the engagement, which sent Marni over the edge.

Melissa Dessent: She was hysterical, absolutely hysterical, crying, inconsolable. And her and her mother cooked up this story that somehow managed to satisfy him and keep him with her. She called me and said to me, "If he calls you or shows up at your house, don't tell him anything."

Yen came back...they married in 1990, but at Marni's own bachelorette party at a club with male strippers...Melissa says marni was up to her old tricks.

Melissa Dessent: Her and a stripper had apparently taken a liking to each other and he invited her into the back room after he was done dancing for free sex backstage. She became obsessed with this stripper and after she had come back from her honeymoon, she had wanted to go back and have sex with him again.

Melissa says Marni didn't go back to the stripper, but she was so disgusted by Marni's behavior and her lying to Yen...Melissa ended their friendship and eventually lost touch with her.

Marni and Yen divorced in 1997. Marni was already working in real estate and later helped Shaun Gayle make a deal. And now police were learning more about Marni's relationship with Shaun. A caller told them it was Marni who had written the angry letters about Shaun...and made them seem as if Monika had. Marni had even boasted that Monika had been threatened with deportation.

Disturbing...reckless behavior, if true...this was the person Shaun Gayle was spending time with. Did he have any idea what she was capable of? Could Marni Yang have become so obsessed with Shaun Gayle...her emotions so distorted...she'd cross the line...and commit murder?

In the weeks after Rhoni Reuter was shot to death, police did not have enough evidence for a warrant to search Marni Yang's house...so they decided to search her trash in the alley...no warrant needed. Inside...unopened credit card bills...which turned out to be a gold mine.

Police Chief Sliozis: Purchases that you wouldn't expect to find on someone's credit card bill, most notably, some books on how to build a silencer.

Rob Stafford: How to build a silencer?

Police Chief Sliozis: Yes. Two books. Paid for with a credit card and overnighted. She actually spent more to have those delivered in an expeditious manner, if you will, than she did for the books themselves.

A recipe for a homemade silencer with ingredients found at any hardware store. And detectives noticed a purchase from Home Depot.

Police Chief Sliozis: As they investigated the receipts through the store, they found out it was the same materials that are listed in the book.

Police Chief Sliozis: We didn't ask the kids. But the investigators knew that the books were purchased during the summer. School was not in session. So what she was saying just didn't make sense. She later changed her story that she actually bought those as a gift for an acquaintance of hers.

Police say Marni didn't say much...but she did offer an alibi.

Police Chief Sliozis: She was home sleeping–went to bed early that night, got up late the next morning, the actual time of the homicide and was home during the entire series of events that we're investigating.

Rob Stafford: Did you ask the children about the alibi?

Police Chief Sliozis: Yes. The son said he was home but never saw her in or about the house.

The older son did not back his mom's alibi. That was surprising enough...but they were startled by what he said next.

Police Chief Sliozis: He told the investigators that after hearing and seeing the story on TV some of his friends brought it to their attention that one of his gut reactions is that she somehow might be involved.

Rob Stafford: His own mother might be involved in the murder of Rhoni Reuter?

Police Chief Sliozis: Correct. Very unusual response. And for a son to do that, I think, every investigator was thinking that's an honest response. The son would have no other reason to answer that way.

Strong circumstantial evidence...but nothing to tie Marni Yang directly to homicide. Police had to release her without filing any charges. For months, as they tried to build a stronger case, police kept an eye on Marni to make sure she didn't flee the country. They also kept her identity a secret.

Rhoni Reuter's family didn't know it was a woman...Shaun Gayle had been seeing...who might be the killer...and that the police were closing in on her.

Wayde Reuter: Dad would always say, "Well, in your day, you might see the person caught. But in my time, I'll never see this happen. I'll never see him caught.

Police had more than an idea...in fact, they thought they had enough evidence to arrest Marni Yang...a pattern of jealous threats...a relationship with Shaun Gayle...ties to a 9mm handgun, and books on how to make a silencer. But that was not enough for the prosecutor in the case, Lake County, Illinois state attorney Michael Waller.

Attorney Waller: One of the questions I asked was, "Can we put Marni Yang anywhere near Deerfield on the October morning of the murder?" And they said, "No," and I said, "Okay, well, that's a problem."

And there was another problem.

Attorney Waller: We don't have any physical evidence tying her to the scene.

Rob Stafford: Blood, hair anything like that?

Attorney Waller: No, there's no physical evidence.

They had only half of the twin pillars of a circumstantial case: motive...Marni's anger at how she badly she felt Shaun treated her...and jealousy of Rhoni Reuter, his most public girlfriend, who would soon have his baby. But if they had motive, they lacked proof Marni Yang had the opportunity to commit the crime.

Police Chief Sliozis: So there was that gray area where we had several conversations, "What more do you need? What can we try? Where do we go from here?"

Attorney Waller: And when we discussed it, we said, "You know, have you checked all potential avenues–you know rental cars–disposable cell phones?" Which is what they did.

Police searched a rental car database and found that even though Marni owned a car, she'd rented a black Volkswagen and returned it a few hours after the homicide...the same color car seen at the crime scene. What's more, a check of gas station surveillance video showed a black VW coming and going near Rhoni's condo around the time of the murder. And one more piece of evidence...the rental record listed a contact number...which detectives discovered was for a disposable cell phone bought at a Wal-Mart a week earlier by Marni Yang.

Attorney Waller: And there was at least one call that placed her in the vicinity of Deerfield on the day of the murder.

Rob Stafford: She has motive. Now you have opportunity.

Attorney Waller: Right.

But the prosecutor wanted even more. It was time for a sting.

Almost a year had passed since Rhoni Reuter and her unborn baby were killed...and still no arrest. The prosecutor wanted more proof Marni Yang had committed the crime before she could be arrested. Investigators knew Marni had a history of sharing private details with friends...so the Deerfield police...along with the Lake county, Illinois major crimes task force...decided to try to catch Marni talking about the shooting. After spending months preparing a wiretap, they brought in one of Marni's closest friends...who'd been questioned before...soon after the murder.

Police Chief Sliozis: I think the gut reactions from the investigators were that if Marni told anybody, this was gonna be the individual. And I think their sense was that–that she knew more than she was letting on.

In fact, Dateline has learned Marni told her friend she'd thought about killing Shaun, the object of her anger, but changed her mind and stuck with her plot to kill only Rhoni, the object of her jealousy. And now that friend would...at last...spill the beans to police.

Police Chief Sliozis: After a pretty extensive interview, several hours, she finally did admit that Marni had discussed with her a plan to kill her, to kill Rhoni Reuter. And then also discussed with her, after the fact, how she did it and what she did.

And the story she told was a bombshell...the black teenager seen leaving the scene of the crime was really Marni Yang, she said...wearing a corn row wig, baggy clothes and black makeup. And she said Marni told her she'd encased the murder weapon in cement and thrown it in a dumpster. And there was another detail police say only the killer could know.

Police Chief Sliozis: Marni told her that when she was leaving the murder scene that Rhoni's legs were in the way, so she had to reach down and push those outta the way to close the door.

Police asked the friend if she'd wear a wire and talk face to face with Marni Yang.

Wearing a wire...in the first operation of its kind in Lake County, Illinois...the friend met Marni at a Denny's restaurant...with unmarked police cars outside and undercover officers inside, investigators listened in and heard Marni describe them as "stupid suburban morons." She also talked about the killing of Rhoni Reuter.

Police Chief Sliozis: She describes how Rhoni opened the door and how she opened fire on her. So again, she was getting into some detail that only that shooter would know.

Powerful evidence...the prosecutor has not yet released to the public...but one other piece was even more damning. Investigators say Marni had stolen something from Rhoni's condo that morning and buried it. With the friend's help, police located the spot...and found the item...which would physically tie Marni to the crime scene. It was something personal...a bracelet like this...

Police Chief Sliozis: Small, beaded bracelet with the word "pregnant." A friend later identified that bracelet as belonging to Rhoni.

Rob Stafford: Case closed?

Police Chief Sliozis: It was probably the definitive piece of evidence that the investigators have been looking for for 16, 17 months.

Police arrested Marni at her home the morning after the meeting at Denny's. On that day in early March, Marni Yang had little to say.

Attorney Waller: In the end, Marni Yang's own words led to her being arrested for first-degree murder.

Prosecutors charged Marni with multiple counts of murder...for killing Rhoni and her unborn child, Skylar. She's pleaded not guilty and is being held without bail. Her attorney did not return repeated phone calls, but he has said previously Marni yang will "vigorously defend herself" at trial.

Attorney Waller: It was an assassination. Her motive was jealousy. As much as it must have enraged her, she was able to carry out this crime and plan it, and cover it up in a very calculating and cold-blooded fashion.

The arrest brought the Reuters relief...but also astonishment...that the person accused of murder...was a woman...a mother of three. Still...Rhoni's family is haunted by what she went through in the final moments of her life.

Thad Reuter: The worst for me is knowing that Rhoni opened that door and saw a person standing there with a gun and screamed, knowing that she–at that moment–knew she was in danger.

The Reuters say they also struggle when they think about the web of relationships and warped emotions that led to Rhoni's door.

Shaun, who cooperated fully with the investigation and provided police his help, says he's still mourning the deaths of Rhoni and the daughter they were to have. He declined our request for an interview...but after Marni's arrest, he publicly denied he and Marni were dating...only that she'd apparently had a crush on him, which he says he knew nothing about.

Shaun Gayle outside courthouse: When I really sit down and just think that this all happened because someone had a crush, it makes no sense to me. She never expressed a desire, an overt desire, for more than our friendship. Other than the occasional attempt to invite me to a social function, which I always turned down because it wasn't that type of a relationship. If I had given her the dating relationship that she wanted, this tragedy may not have happened. This was based on trying to get Rhoni out of the way so she could possibly foster a relationship.

Tangled relationships. Toxic emotions. Twisted actions. How well do we really know the people in our lives? The full truth in this case should come out at Marni Yang's trial. No date has been set...but when it is...Rhoni's brothers say they will be there for the niece they will never know and the sister they will never forget.

Wayde Reuter: I want her to look right at me because I'm gonna look right at her. And I'm gonna stare a hole right through her like it's a bullet hole.

Dateline wires the home of a volunteer, Jenny, from top to bottom with hidden cameras. Then she called repairmen to her house for a simple problem we created as a test with Jenny's pool. NBC's Chris Hansen reports.